Film extrusion die

ABSTRACT

An extrusion die including a pair of lips which together serve to shape a molten thermoplastic polymeric material into a film, with a comparatively short heat exchanger movably mounted on one of such lips for use in selectively varying the temperature at localized areas thereof.

United States Patent [1 1 Mihalik 1 Oct. 23, 1973 1 1 FILM EXTRUSION DIE[75] Inventor: John A. Mihalik, Swarthmore, Pa.

[73] Assignee: FMC Corporation, Philadelphia, Pa.

[22] Filed: July 22, 1971 [21] App]. No.: 165,173

2,861,900 11/1958 Smith et a1 239/1323 X 2,902,716 9/1959 Colombo425/378 X 3,129,459 4/1964 Kullgreen et a1. 425/378 X 3,219,786 11/1965Wenzel 425/378 X Primary ExaminerJ. Spencer Overholser AssistantExaminer-Michael 0. Sutton Att0rneyThomas R. OMalley et a1.

[57] ABSTRACT 7 Claims, 5 Drawing Figures [52] U.S. Cl. 425/378, 425/461[51] Int. Cl B29f 3/04 [58] Field of Search 425/378, 379, 461;239/130,132 132.1,132.3, 135

[56] References Cited UNITED STATES PATENTS 2,937,402 5/1960 Pierce425/378 X 3,497,915 3/1970 R0nden..... 425/378 X 3,069,727 12/1962Shramek 425/379 X PAIENTEU I10! 23 I973 FILM EXTRUSION DIE Thisinvention relates to an extrusion die for use in making films havingimproved gauge characteristics.

Typically, in the manufacture of films from thermoplastic polymericmaterials by extrusion, a polymeric material is rendered molten with ascrew extruder, delivered under pressure to an extrusion die, and forcedfrom between a pair of die lips or blades which together serve to shapethe same into a film. As this shaped polymeric material issues from theextrusion die it is generally drawn down and quenched, as on a chilledroller or within a bath of cool water.

Not uncommon in films formed by such conventional extrusion procedure isthe presence of longitudinally extending gauge bands; that is,lengthwise thickness variations at intervals-across the film width. Suchvariations may be due, for example, totemperature differences along thedie lips, fabrication tolerences, lip warpage which has become set,uneven die lip expansion as the lips are heated during extrusionoperations, etc.

An added source of such variations, and one which appears to haveheretofore been overlooked, is the introduction of temperaturedifferences into the mass of molten polymeric material during itspassage to and/or through the extrusion die. More particularly, while asubstantially uniform temperature may exist throughout the mass ofmolten polymeric material as it leaves the screw extruder, it appearsthat the temperature of portions of such mass is changed during travelthrough a flow line between the extruder and die and/or during travelthrough the die itself, notwithstanding the insulation and/or heatersgenerally provided along such flow line and die. Thus, while the spacingbetween lips of an extrusion die may be substantially uniform, portionsof the molten polymeric material which are at different temperatureswill issue therefrom at different rates and, also, will subject the dielips to uneven heating, with either of these effects being capable ofinducing significant variations in the gauge of the film which is beingproduced.

With conventional elongated extrusion dies, various forms of adjustingmechanisms have been proposed for flexing one or both of the die lips tocorrect for film gauge variations However, such die lips are generallymassive, so as topossess a necessary rigidity under the extrusionpressures which are encountered, and thus respond to adjustments byflexing along a substantial length rather than at selected, localizedareas thereof. Moreover, die-blade flexing is achieved while the die isin a heated condition and usually involves a number of trial-and-erroradjustments so that such procedure is slow and tedious. Accordingly, aprimary object isto provide a new or generally improved and moresatisfactory film extrusion die.

Another object is the provision of an improved extrusion die for use inmaking film from molten polymeric materials which is adapted to becompensated for temperature differentials in the molten polymericmaterial which is being extruded to thereby afford better and moreaccurate control over the gauge of the resulting film.

Still another object is to provide a film extrusion die which includes apair of die lips, with means for varying the temperature of at least oneof such lips at selected localized areas thereof.

Afurther object is the provision of a film extrusion die which includesa pair of die lips and a relatively short heat-exchanger for varying thetemperature of one of the die blades at a localized area thereof.

A still further object is the provision of a film extrusion die having aheat-exchanger which is adjustable along the die lip for use inregulating the temperature at any selected location along the lengththereof.

For a greater understanding of this invention, reference is made to thefollowing detailed description and drawing in which FIG. 1 is adiagrammatic side view of a conventional film extrusion system, withportions thereof shown in part;

FIG. 2 is a front view of one embodiment of the extrusion die of thepresent invention;

FIG. 3 is a transverse vertical section taken along the line III-III ofFIG. 2;

FIG. 4 is a perspective view of a heat-exchanger employed in theextrusion die of the present invention; and

FIG. 5 is a fragmentary view of a modified embodiment of the extrusiondie of the present invention.

The above and other objects of the invention are achieved by a filmextrusion die for use with molten thermoplastic polymeric materialswhich includes a pair of die lips positioned with opposing surfacesthereof in spaced apart relationship to together define an extrusionopening, means for delivering a'molten.

thermoplastic polymeric material under pressure inbetween the die lipsfor extrusion as a continuous film from the extrusion opening, and aheat exchanger adjustably mounted on one of the die lips for movementalong the length thereof into selected positions which are adjacent tobut spaced from the extrusion opening. Compared with the die lips, theheat exchanger is relatively short in length, as for examplethree-eighths inch or less to perhaps three-fourths inch or more, sothat it is capable of varying the temperature of the one die lip at onlylocalized areas thereof.

A heat exchanger suitable for use in the die of the present inventionmay be comprised of a short closed tube with means for circulating aheat exchange medium therethrough. In such heat exchanger, the heatexchange medium may be delivered to and withdrawn directly from theclosed tube. Alternatively, a heat exchange medium may be circulatedthrough a coil which is itself positioned within a heat exchange mediumheld captive within the closed tube. Depending upon the particulareffect which is desired upon the die lip, the exchange medium may beheated or cooled prior to its passage through the closed tube. If theheat exchanger is intended to only apply heat to the extrusion die, anelectrical resistance unit may be employed.

The heat exchanger may be mounted on the die lip by a variety of meanswhich provide for good heat transfer yet facilitateselective movement ofthe heat exchanger into different positions along the length of the dielip. For example, such heat exchanger may be clamped or magneticallyattached to the die lip or, as hereafter described in detail, byproviding a continuous slot along a surface of the die lip remote fromthe extrusion opening. Such slot is shaped to snugly, yet slidablyreceive the particular heat-exchanger employed and, desirably, is formedwith undercut portions so that the heat exchanger may be retained withinsuch slot.

One or more heat exchangers may be provided along one or both of theextrusion die lips. When a plurality of such heat exchangers areemployed, they may be of different lengths and may provide the same ordifferent effects; that is, they may all serve to heat or cool localizedareas of the die lip or lips or some may effect cooling while others areheating different localized die lip areas.

The die lips may be formed integrally with the remainder of otherportions of the extrusion die or one or both of such die lips may bereleasably and adjustably attached to the remainder of the extrusion dieto facilitate variation in the spacing therebetween, at least alongportions of the extrusion opening.

Thermoplastic polymeric materials which are normally shaped into filmsby extrusion may be employed with the extrusion die of the presentinvention. Such film-forming thermoplastic polymeric materials arenumerous and include, for example, polypropylene, polyethylene,polyesters, polyvinyl chloride, etc.

With reference to the drawing, FIG. 1 illustrates a typical filmextrusion system which includes a screw extruder 11 in which athermoplastic polymeric material is rendered molten concomitantly withits advancement by the application of pressure thereto. From theextruder 1 l the molten thermoplastic polymeric material, which may atthis stage be at a substantially uniform temperature, is deliveredthrough conduits l3 and and elbow 17 to a film extrusion die 19. Flanges21 are provided at the ends of the conduits 13 and 15 and the elbow 17to facilitate attachment thereof to each other and to the extruder 11and die 19.

The conduits l3 and 15 and the elbow 17 are generally encased with athermal insulating material, not shown, and heaters, not shown, may alsobe placed at certain areas of the conduits 13 and 15 and/or the elbow17, as well as along the die itself, in an effort to prevent heat lossfrom the material which is being conveyed therethrough. Notwithstandingthese efforts, it appears that differential cooling or heating doesoccur within the mass of molten thermoplastic material during itstransit through the conduits l3 and 15 and elbow 17 and/or the die 19.

Heat losses and gains through the walls and/or flanges of the conduits13 and 15 and elbow 17 may not be uniform and are aggravated by the factthat all portions of the molten thermoplastic material do not travelpaths of equal length between the extruder and die, as during passagethrough the elbow 17 and also throughout die 19. Thus, the portions ofthe molten thermoplastic polymeric material which are nearest to thewalls of the conduits l3 and 15 and elbow 17 may themselves differ intemperature and may be hotter or cooler than the molten material whichflows centrally of such conduits and elbow.

Unfortunately, negligible mixing of the molten thermoplastic materialoccurs during its transit to the die 19 so that temperaturedifferentials are retained by such material during its flow into andthrough the die 19. As a result, all portions of the moltenthermoplastic material do not flow from the die opening at the samerate. Additionally, the lips of the extrusion die are themselvessubjected to different temperatures along the lengths thereof which mayinduce uneven expansion and variation in the spacing between such lips,resulting in longitudinally extending thickness variations in the filmwhich is produced.

Illustrated in FIGS. 2 and 3 of the drawing is an extrusion die 23 ofthe present invention which is adapted to be employed in an extrusionsystem as shown in FIG. 1. The die 23 includes a die body 25 havingalong the length thereof a central cavity 27, into which a moltenthermoplastic material is delivered under pressure, an exposed recess29, and a slot 31 extending between such cavity and recess.

Die lips or blades 33 and 35 are disposed within the recess 29 and areeach fixed to the die body 25 by a separate series of socket head capscrews 37. Openings 39 are provided in the die lips 33 and 35 forpassage of the cap screws 37, with such openings 39 being slightlylarger in diameter than the shanks of the cap screws to permitadjustment of the die lips relative to the die body. As in knownextrusion dies, conventional differential adjusting screw assemblies 41are threaded into the die body 25 and the respective die lips 33 and 35for moving such lips to provide an extrusion opening 43 of desiredwidth.

Extending along an outside surface of at least one of the die lips 33and 35 is a groove 45 which is preferably undercut for retaining thereinone or more heatexchangers 47. As illustrated the groove 45 is locatedadjacent to but spaced from and substantially parallel to the extrusionopening 43 and extends along the length of the die lip.

The particular heat-exchanger 47 shown in FIG. 4 merely consists of atubular body 49 which is closed at its ends 51 and into which opensupply and exhaust conduit 53 and 55. A heat-exchanger medium, as forexample oil, is delivered from a suitable source, not shown, passedthrough the heat exchanger 47 to effect a desired heating or coolingalong the area of the die lip adjacent thereto, and then discharged aswaste or returned to its source where it is again heated or cooled andrecirculated. Desirably, the heat-exchanger 47 makes snug contact withthe die lip, when the latter is in-its heated condition, to provide forgood thermal conduction therebetween and to permit the heatexchanger 47to be retained in its adjusted position merely by the friction betweenthese members.

As heretofore mentioned, the heat-exchanger 47 is relatively short inlength, as compared to the length of die lips 33 and 35, since it isdesired that the influence exerted by such exchanger be confined only toa very small selected area of a die lip. While the width of the filminfluenced by the heat-exchanger is primarily dependent upon the lengthof the heat-exchanger itself, some small variation in such width can beachieved by varying the temperature of the heat exchange mediumcirculated through such heat-exchanger.

As shown in FIG. 1, the extrusion die 23 also includes end plates 57which close off the ends of the central cavity 27, the extrusion opening43, and the groove 45. Unless the opening into the groove 45 at thesurface of the die lip is sufficiently large to permit insertion of theheat-exchanger 47 therethrough, the exchanger is positioned within suchgroove 45 from one of its ends prior to the application of the end plate57 thereat. As illustrated in FIGS. 2 and 3, like grooves 45 may beprovided along both of the die lips 33 and 35 and may each contain asingle or a plurality of separate heatexchangers.

The teachings of the present invention are also applicable for use inthe manufacture of tubular film. Thus, shown in FIG. 5 are annular dielips 59 and 61 which together define an extrusion opening 63 of desiredwidth. A continuous groove 65 is formed in the lip 61 and asillustrated, is substantially parallel to the extrusion opening 63. Thisgroove 65 is undercut, except at the location indicated at 67, to permitinsertion therein of a heat exchanger 69. A plate 71 is applied over thelocation 67 after the heat-exchanger 69 is inserted within the groove65. Operation and function of the heat exchanger 69 is substantially thesame as the exchanger 47 heretofore described.

It is to be understood that changes and variations may be made withoutdeparting from the spirit and scope of the invention as defined in theappended claims.

I claim:

1. A film extrusion die which includes a pair of die lips positionedwith opposing surfaces thereof in spaced apart relationship to togetherdefine an extrusion opening, means for delivering a molten thermoplasticpolymeric material under pressure inbetween said die lips for extrusionas a film from said extrusion opening, and a heat exchanger slidablymounted on one of said die lips for selective adjustment relative tosaid one die lip along a path which is substantially parallel to butspaced from said extrusion opening, said heat exchanger being of suchsize as to contact with only a fractional portion of such one die lip ineach of the positions to which it is adjusted to thereby facilitatevariation in the temperature of said one die lip at only localized areasthereof.

2. A film extrusion die as defined in claim 1 wherein a plurality ofseparate heat exchangers are slidably mounted on said one die lip forindependent sliding adjustment into different positions relative to saidextrusion opening.

3. A film extrusion die as defined in claim 1 wherein at least one heatexchanger is slidably mounted on the other of said die lips forselective adjustment along a path which is substantially parallel to butspaced from said extrusion opening.

4. A film extrusion die as defined in claim 1 wherein said heatexchanger includes a closed tube and means for circulating a heatexchange medium through such tube.

5. A film extrusion die as defined in claim 4 wherein said one die lipincludes a continuous slot formed in a surface thereof remote from saidopposing surfaces, said slot shaped to slidably receive said heatexchanger and having undercut portions to retain said heat exchangertherein.

6. A film extrusion die as defined in claim 5 wherein said die lips areannular for use in forming tubular films.

7. A film extrusion die as defined in claim 5 wherein said die lips areelongated for use in forming flat films.

1. A film extrusion die which includes a pair of die lips positionedwith opposing surfaces thereof in spaced apart relationship to togetherdefine an extrusion opening, means for delivering a molten thermoplasticpolymeric material under pressure inbetween said die lips for extrusionas a film from said extrusion opening, and a heat exchanger slidablymounted on one of said die lips for selective adjustment relative tosaid one die lip along a path which is substantially parallel to butspaced from said extrusion opening, said heat exchanger being of suchsize as to contact with only a fractional portion of such one die lip ineach of the positions to which it is adjusted to thereby facilitatevariation in the temperature of said one die lip At only localized areasthereof.
 2. A film extrusion die as defined in claim 1 wherein aplurality of separate heat exchangers are slidably mounted on said onedie lip for independent sliding adjustment into different positionsrelative to said extrusion opening.
 3. A film extrusion die as definedin claim 1 wherein at least one heat exchanger is slidably mounted onthe other of said die lips for selective adjustment along a path whichis substantially parallel to but spaced from said extrusion opening. 4.A film extrusion die as defined in claim 1 wherein said heat exchangerincludes a closed tube and means for circulating a heat exchange mediumthrough such tube.
 5. A film extrusion die as defined in claim 4 whereinsaid one die lip includes a continuous slot formed in a surface thereofremote from said opposing surfaces, said slot shaped to slidably receivesaid heat exchanger and having undercut portions to retain said heatexchanger therein.
 6. A film extrusion die as defined in claim 5 whereinsaid die lips are annular for use in forming tubular films.
 7. A filmextrusion die as defined in claim 5 wherein said die lips are elongatedfor use in forming flat films.